A Watertown resident, Andrew Kitzenberg, 29, said he looked out his third-floor window to see two young men of slight build engaged in constant gunfire with police officers. A police vehicle drove towards the shooters, he said, and was shot at until it was severely damaged. It rolled out of control, Mr. Kitzenberg said, and crashed into two cars in his driveway. The gunmen, he said, had a large, unwieldy bomb that he said looked like a pressure cooker.
They lit it, still in the middle of the gunfire, and threw it, he said. But it went 20 yards at most. It exploded, he said, and one man ran toward the gathered police officers. He was tackled, but it was not clear if he was shot, Mr. Kitzenberg said.